Transport of organic substances in the phloem

• Translocation - The process by which organic molecules and some mineral ions are transported from one part of a plant to another
• Translocation is the process by which products of photosynthesis are transported from a source (where they are made - usually leaves) to a sink (a tissue that needs them)
• Translocation is carried out by the phloem tissue which is made up of sieve-tube elements and their companion cells
• Having produced sugars during photosynthesis, the plant transports them from the sites of production, known as sources, to the places where they will be used directly or stored for future use - known as sinks
• Translocation is bidirectional:

  • As sinks can be anywhere in a plant - sometimes above and sometimes below the source - translocation of molecules in phloem can be in either direction
• Source:

  • Where solutes are produced
  • High concentration
  • Eg: Sucrose in leaves
• Sink:

  • Where solutes are used up
  • Lower concentration
  • Eg: Sucrose in potatoes
• Translocation is bidirectional so must as be an active process which requires energy as the solutes (sucrose and other organic molecules) are pushed around under high pressure